The skeletal system includes many long bones that extend from the human torso. These long bones include the femur, fibula, tibia, humerus, radius and ulna.
A joint within the human body forms a juncture between two or more bones or other skeletal parts. The ankle, hip, knee, shoulder, elbow and wrist are just a few examples of the multitude of joints found within the body. As should be apparent from the above list of examples of joints, many of the joints permit relative motion between the bones. For example, the motion of sliding, gliding, and hinge or ball and socket movements may be had by a joint. For example, the ankle permits a hinge movement, the knee allows for a combination of gliding and hinge movements and the shoulder and hip permit movement through a ball and socket arrangement.
The joints in the body are stressed or can be damaged in a variety of ways. For example, gradual wear and tear is imposed on the joints through the continuous use of a joint over the years. The joints that permit motion have cartilage positioned between the bones providing lubrication to the motion and also absorbing some of the forces direct to the joint. Over time, the normal use of a joint may wear down the cartilage and bring the moving bones in a direct contact with each other. In contrast, in normal use, a trauma to a joint, such as the delivery of a large force, from an accident, for example, an automobile accident, may cause considerable damage to the bones, the cartilage or to other connective tissue such as tendons or ligaments.
Arthropathy, a term referring to a disease of the joint, is another way in which a joint may become damaged. Perhaps the best known joint disease is arthritis, which is generally referred to a disease or inflammation of a joint that results in pain, swelling, stiffness, instability, and often deformity.
There are many different forms of arthritis, with osteoarthritis being the most common and resulting from the wear and tear of a cartilage within a joint. Another type of arthritis is osteonecrosis, which is caused by the death of a part of the bone due to loss of blood supply. Other types of arthritis are caused by trauma to the joint while others, such as rheumatoid arthritis, Lupus, and psoriatic arthritis destroy cartilage and are associated with the inflammation of the joint lining.
The hip joint is one of the joints that is commonly afflicted with arthropathy. The hip joint is a ball and socket joint that joins the femur or thighbone with the pelvis. The pelvis has a semispherical socket called the acetabulum for receiving a ball socket head in the femur. Both the head of the femur and the acetabulum are coated with cartilage for allowing the femur to move easily within the pelvis. Other joints commonly afflicted with arthropathy include the spine, knee, shoulder, carpals, metacarpals, and phalanges of the hand.
Arthroplasty as opposed to arthropathy commonly refers to the making of an artificial joint. In severe cases of arthritis or other forms of arthropathy, such as when pain is overwhelming or when a joint has a limited range of mobility, a partial or total replacement of the joint within an artificial joint may be justified. The procedure for replacing the joint varies, of course, with the particular joint in question, but in general involves replacing a terminal portion of an afflicted bone with a prosthetic implant and inserting a member to serve as a substitute for the cartilage.
The prosthetic implant is formed of a rigid material that becomes bonded with the bone and provides strength and rigidity to the joint and the cartilage substitute members chosen to provide lubrication to the joint and to absorb some of the compressive forces. Suitable materials for the implant include metals and composite materials such as titanium, cobalt chromium, stainless steel, ceramic and suitable materials for cartilage substitutes include polyethylene, ceramics, and metals. A cement may also be used to secure the prosthetic implant to the host bone.
A total hip replacement, for example, involves removing the ball shaped head of the femur and inserting a stem implant into the center of the bone, which is referred to as the medullary canal, or marrow of the bone. The stem implant may be cemented into the medullary canal or may have a porous coated surface for allowing the bone to heal directly to the implant. The stem implant has a neck and a ball shaped head, which are intended to perform the same functions as a healthy femur's neck and a ball shaped head. The polyethylene cup is inserted into the acetabulum and has a socket for receiving the head on the stem implant.
The polyethylene cup may be positioned directly into the acetabulum. Preferably, the polyethylene cup is secured to a metal member, which is in turn secured to the acetabulum. This metal member is typically called a cup or a shell. The cup or shell may include a porous coating for promoting bony in-growth to secure the shell to the acetabulum. Alternatively or in addition the shell may include an opening or a plurality of openings for receiving bone screws to assist in the attachment of the shell to the acetabulum. As an alternative to the polyethylene cup, a cup of a different material may be inserted into the shell. For example, the cup may be made of a metal, for example, cobalt chromium, stainless steel, or titanium. Alternatively, the cup may be made of a ceramic.
More recently, the polyethelene cup as a hip cup prosthesis has been replaced with a more rigid component. For example, in more recent hip cup prostheses, the cup is made of, for example, a metal or a ceramic. The head may be made of a metal or a ceramic. For example, the cup may be made of a ceramic and the head may likewise be made of a ceramic. Alternatively, the cup may be made of a metal and the head may likewise be made of that similar metal. It should be appreciated that a ceramic cup may be utilized with a metal head and a metal cup may be utilized with a ceramic head.
To maximize the life of the prosthesis, the accuracy of the dimensional characteristics of the components of the prosthesis as well as the surface condition, for example the surface finish, is extremely critical in the life of the prosthesis. Dimensional errors and surface finish imperfections may cause the prosthesis to prematurely wear. The components that wear on the prosthesis, particularly those that wear rapidly, may lead to reactions with the tissues of the body. Such reaction to foreign objects is called osteolysis. Osteolysis can damage soft tissue and further complicate the replacement of the prosthesis.
Attempts have been made to provide for improved finishes and geometries of the articulating surface of a prosthesis. For example, the surfaces may be polished by hand by, for example, a rubbing compound or by a metal or cloth buffing wheel. Alternatively, the surfaces may be smoothed by robotic manipulators using similar tools as are used by hand. Alternatively, the components have the articulating surface of the prosthesis may be polished by a finishing device, for example a RotoFinish® tumbling machine. These prior art attempts at providing improved geometry and finish to the articulating surface of a prosthetic component are slow and inaccurate. Further, the use of the continual attempts to improve the finish on the part may affect its geometry or shape. Imperfections in shape and or finish may greatly reduce the operating life of the prosthesis and may lead to osteolysis.
The present invention is adapted to solve at least some of the aforementioned problems with the prior art.